1. Field of the Invention
The present invention relates generally to motor control devices for controlling the direction of rotation and the braking of a DC motor, and more particularly to devices useful for controlling a motor employed in an automatic focusing apparatus or an automatic diaphragm setting apparatus in a camera.
2. Description of the Prior Art
In various apparatus employing a DC motor (hereinafter referred to simply as a motor) driven from a DC power source, it is generally required not only to control the rotation of the motor in one direction, but also to stop the motor instantaneously and to reverse its direction of rotation. The ability to instantaneously stop a motor and quickly change its rotational direction, is essential when the motor is employed in a servo system, where rapid and precise control are generally important.
Within limits, a servo system having some of the above-mentioned characteristics may be obtained by increasing the sensitivity or the responsiveness of the servo system. However, increased sensitivity enhances the hunting phenomenon caused by the inertia of the motor and the driven members, and results in reduced stability in operation. Many attempts have been made to lessen the hunting phenomenon. One approach is to increase the reduction ratio of a gear train which is driven by the shaft of the motor. Another known approach is to broaden the dead range of the servo system. However, all the known approaches either unduly limit the sensitivity of the servo system and/or reduce the precision with which the system can be controlled.
Moreover, even for non-servo systems or apparatus which simply use a motor as a driving source, where the inertia of driven members, such as torque transmission members, for example, engaged by the shaft of the motor is large, it is very difficult to stop the rotation of the motor instantaneously or to exercise precise control over its rotation.
Thus, all of the apparatus requiring rapid and precise control of a motor, particularly those apparatus employing a miniature DC motor, such as in a servo mechanism for an automatic diaphragm setting apparatus or an automatic focusing apparatus in a camera, always involve the above-mentioned problems.